Attention has been given to lithium ion secondary batteries as a power source for driving electronic equipment. Negative electrodes for lithium ion secondary batteries comprising a graphite material have an average potential during the desorption of lithium ions of about 0.2 V (vs. Li/Li+) and exhibit a relatively flat potential. This potential is lower than that of negative electrodes comprising hard carbon, and therefore equipment that requires high voltage and voltage flatness currently employs, as the power source, lithium ion secondary batteries comprising negative electrodes including a graphite material. Graphite materials, however, have a small capacity per unit weight of 372 mAh/g, and a further increase in capacity cannot be expected.
Meanwhile, materials capable of forming an intermetallic compound with lithium are considered promising as negative electrode materials which provide a high capacity. Such materials include silicon, tin and oxides thereof. During the absorption of lithium ions, however, the crystal structure of these materials changes so that the volume of the materials increases. In the case of a negative electrode including an active material consisted of Si, the negative electrode active material is represented by Li4.4Si in the state where the maximum amount of lithium ions is absorbed. When Si changes into Li4.4Si, the volume increases by 4.12 times. In the case of graphite, on the other hand, even if the maximum amount of lithium ions is absorbed, its volume increases only by 1.2 times.
A large volume change of active material results in cracking of active material particles, insufficient contact between active material and current collector, etc, which shortens charge/discharge cycle life. Particularly when cracking of active material particles occurs, the surface area of the active material particles increases, and the reaction between the active material particles and a non-aqueous electrolyte is accelerated. As a result, a film made of decomposition product of the electrolyte is likely to be formed on the surface of the active material. The formation of such film increases the interface resistance between the active material and the electrolyte, which is considered as a major cause for short charge/discharge cycle life.
In order to prevent the deterioration of charge/discharge cycle life, for example, Japanese Laid-Open Patent Publication No. 2002-83594 discloses to form an amorphous silicon thin-film on a current collector having a rough surface so as to create space for relieving the expansion stress, thereby ensuring current collecting efficiency. In order to increase the adhesion strength between the copper current collector and the thin-film, the above-mentioned patent publication (Japanese Laid-Open Patent Publication No. 2002-83594) proposes a method for forming a silicon-copper composite layer by forming an amorphous silicon thin-film on the current collector, followed by heat treatment.
In order to prevent an active material from cracking, for example, Japanese Patent Publication No. 2997741 teaches the use of a negative electrode active material composed of SiOx (0<x<2) having a low expansion coefficient during charge.
In order to improve battery capacity by the reduction of irreversible capacity and to improve cycle characteristics by ensuring conductivity of active material particles, for example, Japanese Laid-Open Patent Publication No. 2004-047404 teaches the use of a negative electrode active material composed of a silicon composite made of silicon oxide particles having silicon fine particles dispersed therein and carbon covering the silicon oxide particles.
In order to enhance charge/discharge efficiency, for example, Japanese Patent Publication No. 3520921 teaches a negative electrode having a multilayered structure composed of a carbon layer and a silicon oxide thin-film layer.
However, the negative electrodes disclosed in the above-mentioned prior art references suffer from various problems. For example, the present inventors examined the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2002-83594 only to find that lithium ion conductivity in the silicon was low, and that polarization increased when charge/discharge was performed at a high current value (i.e., high-load charge/discharge or high rate charge/discharge), resulting in a low discharge capacity. In a silicon thin film, in particular, a large concentration gradient of lithium is produced in the thickness direction, and the capacity decreases easily. Further, because silicon has an extremely large expansion coefficient, a negative electrode comprising silicon is highly deformed so that the electrode group having positive and negative electrodes disposed opposite to each other is buckled.
In order to overcome the above problems, the expansion stress at the interface between silicon and a current collector must be relieved. However, this requires considerable costs because additional steps are necessary such as a step of forming silicon into a columnar structure or a step of performing heat treatment for diffusing copper in the silicon.
As for the negative electrode disclosed by Japanese Patent Publication No. 2997741, because the active material layer is composed of a single-phase SiOx, the conductivity thereof is low. In this case, the capacity density becomes low because the addition of a conductive material such as carbon to the active material layer is necessary. Also, because the irreversible capacity is large, the capacity of the negative electrode is smaller than that of the positive electrode so that the battery capacity becomes significantly low. From these reasons, the negative electrode of Japanese Patent Publication No. 2997741 fails to take advantage of the characteristics of high-capacity silicon and to provide a capacity as expected.
A further problem arises when the negative electrode contains a carbon material as a conductive material: propylene carbonate cannot be used as a solvent for electrolyte, because propylene carbonate is decomposed on the surface of the carbon material.
As for the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2004-047404, because SiOx is heat-treated to prepare silicon microcrystallites, it is difficult to control the size of the microcrystallites. When the microcrystallites have a size exceeding a certain size, cracking occurs during the expansion. In this method, because silicon crystals are inherently produced, it is impossible to form amorphous silicon which is advantageous for absorption and desorption of Li. Besides, such microcrystalline silicon is produced spot-wise, and they are surrounded by SiOx having low conductivity, which leads to a low high rate charge/discharge capacity. Also, because Japanese Laid-Open Patent Publication No. 2004-047404 requires the use of a carbon material, propylene carbonate cannot be used as a solvent for electrolyte, as is the case in Japanese Patent Publication No. 2997741.
Moreover, the negative electrodes disclosed by Japanese Patent Publication No. 2997741 and Japanese Laid-Open Patent Publication No. 2004-047404 are produced by mixing the negative electrode active material, a conventional conductive material and a conventional binder to form a mixture which is then applied to a metal foil. In this case, because the active material particles and the current collector layer are bonded by the binder, the following problem arises: due to the large volume change of the active material during charge/discharge cycles as stated earlier, the conductive material and the binder cannot adjust to the volume change so that during repeated charge/discharge cycles, the contact between the active material and the conductive material as well as that between the active material and the binder cannot be maintained. As a result, the contact between the active material and the current collector is weakened, and the polarization increases, resulting in a low charge/discharge capacity.
The negative electrode disclosed by Japanese Patent Publication No. 3520921 contains, as the negative electrode active material, silicon oxide in which the molar ratio of oxygen relative to silicon is 0 to 2. In a thin film layer made of the silicon oxide, the oxygen ratio is uniform in any portion of the layer. When the silicon oxide has a high oxygen ratio, although the expansion coefficient is small during charge and an excellent lithium ion conductivity is obtained, the charge/discharge capacity is small. Conversely, when the silicon oxide has a low oxygen ratio, although the charge/discharge capacity is large, the expansion coefficient during charge is large, and the lithium ion conductivity is low.
When producing a negative electrode active material layer by vapor deposition method or sputtering method with the use of SiO as the target, Si—O bonds formed from the oxide of divalent silicon exist in the resulting thin-film. When such bonds react with lithium, oxygen is easily reduced and reacts with other lithium. This increases irreversible capacity, resulting in a low battery capacity.
Moreover, because the silicon oxide thin-film layer is in contact with a carbon layer, the carbon layer and the silicon oxide thin-film layer are separated from each other due to expansion stress during charge, resulting in low current collecting efficiency. Further, since the production of the carbon layer and the silicon oxide layer requires a completely different production process, the costs for producing negative electrodes will be very high, and negative electrodes cannot be produced efficiently.
In view of the foregoing, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery having high capacity, superior high rate charge/discharge characteristics and excellent cycle characteristics, and a lithium ion secondary battery comprising the negative electrode.